[Feature]: Mock another contract in #[motsu::test]

[bidzyyys]

What is the feature you would like to see?

Some contracts need to call another contracts, e.g. Erc721::_check_on_erc721_received.
We want to be able to mock some behaviour, or at least mock return data in motsu unit tests.

pub fn _check_on_erc721_received(
        &mut self,
        operator: Address,
        from: Address,
        to: Address,
        token_id: U256,
        data: Bytes,
    ) -> Result<(), Error> {
        const RECEIVER_FN_SELECTOR: FixedBytes<4> = fixed_bytes!("150b7a02");

        if !to.has_code() {
            return Ok(());
        }

        let receiver = IERC721Receiver::new(to);
        let call = Call::new_in(self);
        let result = receiver.on_erc_721_received(
            call,
            operator,
            from,
            token_id,
            data.to_vec().into(),
        );

        let id = match result {
            Ok(id) => id,
            Err(e) => {
                if let call::Error::Revert(ref reason) = e {
                    if reason.len() > 0 {
                        // Non-IERC721Receiver implementer.
                        return Err(Error::InvalidReceiverWithReason(e));
                    }
                }

                return Err(ERC721InvalidReceiver { receiver: to }.into());
            }
        };

        // Token rejected.
        if id != RECEIVER_FN_SELECTOR {
            return Err(ERC721InvalidReceiver { receiver: to }.into());
        }

        Ok(())
    }
}

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